The present invention relates to a structure and a method of mounting an electronic component, more precisely relates to a structure and a method of mounting an electronic component, in which the electronic component is bonded by ultrasonic waves.
When a semiconductor chip, which is an example of the electronic component, is mounted on a circuit board by flip chip bonding, bumps of the semiconductor chip are headed for the circuit board, then the bumps, which are made of gold or solder, are respectively bonded to electrodes of the circuit board. A space between the semiconductor chip and the circuit board is filled with synthetic resin so as to protect a circuit face of the semiconductor chip, prevent the bumps from corrosion and improve bonding strength therebetween.
These days, the semiconductor chip is flip-chip-bonded by applying ultrasonic vibrations. By applying ultrasonic vibrations, bonding sections between the bumps and the electrodes are alloyed and bonded or bonded.
These days, semiconductor chips are miniaturized and have many pins. Therefore, separations between bums must be very short. When the small semiconductor chip having many pins is bonded to a circuit board by ultrasonic vibrations, the bumps of the semiconductor chip are easily displaced from electrodes of the circuit board, so that bonding reliability between the bums and the electrodes is lowered.
FIGS. 7A and 7B show a conventional structure of mounting a semiconductor chip on a circuit board. FIG. 7A is a plan view showing positional relationships between bumps 12a and 12b of the semiconductor chip 10 and electrodes 22a and 22b of the circuit board; FIG. 7B is a partial front view of bonding sections between the semiconductor chip 10 and the circuit board 20. The semiconductor chip 10 is flip-chip-bonded to the circuit board 20 by ultrasonic vibrations. Bumps 12 of the semiconductor chip 10 are displaced from electrodes 22 of the circuit board 20.
By applying ultrasonic vibrations to the semiconductor chip 10, the semiconductor chip 10 reciprocally moves in a direction of the ultrasonic vibrations, e.g., right and left in FIG. 7A. Each of the electrodes 22 is formed into a thin rectangular. Therefore, even if the bumps 12a are displaced from the rectangular electrodes 22a, whose longitudinal axis is parallel to the direction of the ultrasonic vibrations, and bonded to the electrodes 22a, the bonding sections occur no problems. On the other hand, if the bumps 12b are displaced from the rectangular electrodes 22b, whose transverse axis is parallel to the direction of the ultrasonic vibrations, the bumps 12b are bonded at positions displaced from centers of the electrodes 22b, so that bonding reliability between the bums 12b and the electrodes 22b must be lowered.
The inventors measured displacement of semiconductor chips with respect to circuit boards. The results are shown in FIG. 8. Three groups (1), (2) and (3) of samples were prepared. Semiconductor chips of the samples were flip-chip-bonded to the circuit boards. In each group (1), (2) and (3), three samples were flip-chip-bonded with loads of 5, 10 and 15 gf/bump respectively. No ultrasonic vibrations were applied to the samples of the group (1); ultrasonic vibrations of 200 kHz were applied to the group (2); and ultrasonic vibrations of 50 kHz were applied to the group (3). According to FIG. 8, in the case of applying no ultrasonic vibrations, displacement of the semiconductor chips were small without reference to variation of the loads. On the other hand, in the case of applying ultrasonic vibrations, the displacement was easily occurred, and the displacement was increased with increasing frequency of ultrasonic vibrations.
In each group (2) and (3), ultrasonic vibrations having the same frequency were applied to all of the samples. The displacement was increased with reducing the load. Therefore, the load should be small so as not to damage the semiconductor chip, but the displacement of the semiconductor chip must be increased.